Polyolefin resins having bimodal molecular weight distributions and/or bimodal composition distributions are desirable in a number of applications. Resins including a mixture of a relatively higher molecular weight polyolefin and a relatively lower molecular weight polyolefin can be produced to take advantage of the increased strength properties of higher molecular weight resins and articles and films made therefrom, and the better processing characteristics of lower molecular weight resins.
Bimetallic catalysts such as those disclosed in U.S. Pat. Nos. 5,032,562 and 5,525,678, and European Patent EP 0 729 387, can produce bimodal polyolefin resins in a single reactor. These catalysts typically include a non-metallocene catalyst component and a metallocene catalyst component which produce polyolefins having different average molecular weights. U.S. Pat. No. 5,525,678, for example, discloses a bimetallic catalyst in one embodiment including a titanium non-metallocene component which produces a higher molecular weight resin, and a zirconium metallocene component which produces a lower molecular weight resin. Controlling the relative amounts of each catalyst in a reactor, or the relative reactivities of the different catalysts, allows control of the bimodal product resin. Other background references include EP 0 676 418, WO 98/49209, WO 97/35891, and U.S. Pat. No. 5,183,867.
Methods of producing bimetallic catalysts are disclosed in the references cited above. These methods generally include depositing a non-metallocene transition metal compound on a dehydrated porous support, and subsequently depositing a metallocene compound on the same support. For some applications, however, the activity of the known bimetallic catalysts is undesirably low. It would be desirable to have methods of producing bimetallic catalysts for producing bimodal polyolefin resins, which have a higher activity than bimetallic catalysts currently known.